According to conventional technique, thermoplastic foam may be produced by feeding thermoplastic resin into an extruder and, through the shearing action of one or more screws, melting the resin continuously in the barrel of the extruder. In an intermediate or a mixing section, a solvent blowing agent, usually in a liquid or gaseous state, is continuously injected into the molten resin. Alternatively, a chemical blowing agent may be dispersed throughout the particulate resin in powder form before the resin is fed to the extruder as discussed in U.S. Pat. No. 4,107,260. In either case, the extruder screw is designed to mix and dissolve the blowing agent as uniformly as possible in the molten resin. Thorough, uniform mixing is essential to the production of a high quality foam. The resultant mixture must be maintained under carefully controlled temperatures and pressures within the extruder in order to prevent the premature volatilization of the blowing agent. When the molten mixture is forced through a die, the material undergoes decompression to atmospheric pressure so that the blowing agent expands within the body of material as bubbles and a foam is produced.
Recently, the use of solvent blowing agents such as chlorofluorocarbons has come under scrutiny because of a suspected detrimental effect on the atmosphere, particularly on the ozone layer. The use of such blowing agents has become increasingly regulated, and will likely be virtually eliminated in the future. Further, although other solvent blowing agents such as hydrocarbons are believed to be somewhat less deleterious to the atmosphere than chlorofluorocarbons, hydrocarbons have flammability problems and thus their use is hazardous and/or undesirable. Since solvent blowing agents are generally utilized to produce low density foams, i.e., foams having a density less than 0.5 g/cc (31.2 lbs/ft.sup.3), the elimination and/or regulation of solvent blowing agents presents a significant problem to the foam industry.
Alternatives to chlorofluorocarbons and hydrocarbons include nitrogen and carbon dioxide. However, it is difficult to mix nitrogen and carbon dioxide into thermoplastic elastomers.
One particularly attractive commercially available thermoplastic rubber is Santoprene.RTM., available from Advanced Elastomers. Santoprene is a registered trademark of Advanced Elastomers. In a Monsanto Company publication "Extrusion Foaming Technology for Santoprene Thermoplastic Rubber" (May 10, 1985), a method is described for producing extrusions of foamed Santoprene. The above-identified Monsanto publication recommends that to produce a high-density foamed Santoprene, a modified azodicarbonamide chemical foaming agent be used. Fluorocarbon-11 is recommended as a blowing agent to produce a low density foam having a density of 0.2-0.5 g/cc (12.5-31.2 lbs/ft.sup.3). Commonly assigned U.S. Pat. No. 4,898,760 describes a method and apparatus for producing a soft, thermoplastic rubber foam with densities in a range of approximately 0.03-0.30 g/cc (2-19 lbs/ft.sup.3). As discussed in the '760 patent, such a foam is particularly useful as a weatherstrip, for example. However, the preferred blowing agent is a chlorinated fluorocarbon, which suffers from the deficiencies noted above. Further, newly developed blowing agents are typically very expensive to develop and ultimately result in increased processing costs.
Despite extensive research and investment, a need still remains for an inexpensive, environmentally safe, non-flammable blowing agent for thermoplastic elastomers, in particular low density thermoplastic rubber foams.